Method and apparatus for sensing distortion

ABSTRACT

A work holder capable of measuring the amount of distortion occurring in a part as the part is machined, processed, treated, or otherwise operated upon and methods of using the same for quality control and process monitoring purposes. Indirect measurement of the distortion occurring in the part eliminates the need to individual strain gauge each part

CROSS-REFERENCE TO PRIOR APPLICATION

This application claims benefit of Provisional Patent Application Ser.No. 60/579,489, filed Jun. 14, 2004.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for securing a part, such as awork holder or fixture, capable of monitoring, sensing, or measuring themechanical distortion of the part as it is machined, treated orotherwise operated upon.

It is well known that metallic parts, such as aircraft engine parts, maydistort when subjected to processes such as burnishing, shot peening,laser shock peening, or other similar treatments. These distortions arecaused by compressive stresses that are introduced and tensile stressesthat develop in the part as a result of such processes. A part may besubjected to processes such as burnishing, shot peening or laser shockpeening to improve the parts resistance to stress related failuremechanisms. The benefit of such processes is obtained by inducingcompressive stresses in the surface of the part. Metallic materialstreated in such a manner respond to the introduction of compressivestresses by being self-equilibrating. Thus, as compressive stresses areintroduced in the part, reacting tensile stresses form elsewhere in thepart such that all forces within the volume of the part sum to zero.However, as these stresses develop, the material forming the part mayreact by distorting to accommodate the introduction of new stresses.

One problem associated with such distortions is that they go beyond theacceptable engineering tolerances for that particular part. While themechanisms that cause distortion are known and understood, monitoringindividual parts for excessive distortion beyond acceptable engineeringtolerances has been difficult because of the various factors thatcontribute to distortion as well as the complexity of many part designs.Factors that have been found to contribute to distortion include thetype, magnitude and pattern of stress introduced in the part, theparticular method by which the part was treated, the material(s) fromwhich the part is made, and the particular geometry of the part. Anothersubstantial factor which impacts distortion are variations in individualparts, most notably in parts being treated that have already undergonesome service life and are being remanufactured or refurbished. Undersuch circumstances, it becomes even more critical to monitor theresponse of each individual part to the treatment process for qualitycontrol purposes. Furthermore, for parts having complex geometries, suchas gas turbine engine blades, it is difficult to measure the degree towhich the part may have distorted, especially in the context of asizeable manufacturing operation.

Accordingly, until now, it has been impractical or unduly laborious tomeasure individual parts and thereby assess whether a part has beenadequately processed or whether a particular treatment operation hascaused distortion beyond acceptable engineering tolerances. Therefore, aneed exists for a relatively inexpensive method and apparatus fordetermining the distortion of a part as a result of a treatment processand that is easily implemented in a manufacturing environment.

SUMMARY OF THE INVENTION

The subject invention relates to a distortion sensing work holder forsecuring a part during a machining, treatment, or other process that iscapable of monitoring, measuring, recording or otherwise sensing themechanical distortion of the part as it is subjected to the process. Thedistortion sensing work holder is preferably capable of monitoring,sensing, or measuring the mechanical distortion of the part as it ismachined, treated or otherwise operated upon.

By way of description, a preferred embodiment of the distortion sensingwork holder would be used in the following manner: A part to be treatedis clamped in a work holder. Once the part is clamped it is attached toa linkage such that the part is in mechanical communication with anelastically deformable element. The linkage may be attached to the partwith bolts, clamps, and the like, or specially designed grippers whereinthe gripping faces of the gripper correspond to the contours of thespecific part. With the part secured and in mechanical communicationwith the elastically deformable element, the part is treated byburnishing, laser shock peening or other similar surface treatments. Asthe part is treated, distortions in the part that may result from thetreatment process are communicated, via the linkage, to the elasticallydeformable and recoverable element. Strain sensors or deflection sensorsmounted on the deformable element sense the distortion and output asignal corresponding to the magnitude of the distortion. This signal isrecorded throughout the treatment process. Based on this signal, adistortion signature indicative of the distortions experienced by thepart over the course of the treatment procedure is compiled for thatparticular process and part combination. This signature may then beutilized for quality control and process evaluation purposes forsubsequent treatment procedures.

In another preferred embodiment of the invention, the distortion sensingwork holder provides a means for monitoring the repeatability ofmachining, treating, or other procedures as a measure of qualitycontrol.

In another preferred embodiment of the invention, the distortion sensingwork holder is effective for providing a means to monitor a machining,treating, or other process to ensure proper operating conditions for theprocess.

In another preferred embodiment of the invention, the distortion sensingwork holder is effective for providing means for monitoring the progressof a machining, treating, or other process and to provide correctiveaction when the process deviates from acceptable or establishedparameters.

In another preferred embodiment of the invention, the distortion sensingwork holder is effective for providing an apparatus for securing a partduring a machining, treatment, or other process that is capable ofmonitoring, measuring, recording, or otherwise sensing the distortion ofa part as it is subjected to the process, utilizing the output of suchan apparatus to develop a distortion signature for the particularprocess; and utilizing the distortion signature to monitor the processfor abnormal operating conditions; and utilizing the distortionsignature as a quality control measure for individual parts.

In another preferred embodiment of the invention, the distortion sensingwork holder comprises a means for securing a part being treated; arigid, elastically deformable and recoverable material fixedly attachedto the securing means; a means for mechanically linking the part beingoperated upon to the rigid, elastically deformable and recoverablematerial; a means for measuring the strain developed in or thedeflection of the material; and a means for capturing and recording theoutput of the strain or deflection measuring means.

In another preferred embodiment of the present invention, the distortionsensing work holder comprises a means for clamping a part, the clampingmeans having a base integrally formed with a support member forsupporting the base on a conventional tool holder; a rigid, elasticallydeformable and recoverable material in the shape of a beam, the beamhaving a first and second end, the second end being fixedly attached tothe base of the clamping means; strain or deflection sensing meansfixedly attached between the first and second ends of the beam andbetween the beam and the base of the clamping means; a linkage forcommunicating distortions from a part to the beam, the linkage havingtop and bottom ends, the bottom end of the linkage being fixedlyattached to the first end of the beam, the top end of the linkage havingmeans for gripping and releasing the end of a part opposite the meansfor clamping; a means for monitoring, recording over time, and analyzingthe output of the strain or deflection sensing means, the means formonitoring, recording, and analyzing having a human readable display ofthe information collected.

In another preferred embodiment of the invention, the distortion sensingwork holder comprises a means for securing a part being treated; a meansfor measuring the deflection of the part being treated; and a means forcapturing and recording the output of the deflection measuring means.

In another preferred embodiment of the present invention, the distortionsensing work holder comprises a means for clamping a part, the clampingmeans having a base integrally formed with a support member forsupporting the base on a conventional tool holder; selectivelypositionable deflection measuring means; and a means for monitoring,recording, and analyzing the output of the deflection sensing means, themeans for monitoring, recording, and analyzing having a human readabledisplay of the information collected.

Another preferred embodiment of the invention is a method for using adistortion sensing work holder for quality control purposes. The methodcomprises the steps of placing a part to be machined, treated orotherwise operated upon in the distortion sensing work holder; obtaininga distortion signature for the particular operation by recording theoutput of the strain or deflection sensing means as a function of timeor the position of the tooling during treatment; comparing the recordeddistortion signature against a known or verified signature for the sameoperation on the same part; determining if the part is acceptable basedupon the comparison between the known signature and the recordedsignature.

In another preferred embodiment of the present invention, the apparatusis used in conjunction with a method for determining abnormal operationof the equipment operating on the part. The method comprises placing apart to be machined, treated or otherwise operated upon in thedistortion sensing work holder; obtaining a distortion signature for theparticular operation by recording the output of the strain or deflectionsensing means; comparing the recorded distortion signature against aknown or verified signature for the same operation on the same part;determining if the part is acceptable based upon the comparison betweenthe known signature and the recorded signature.

The method and apparatus of the present invention can be used to measurethe distortion occurring in parts with a high length to cross-sectionalarea ratio including, but not limited to, turbine and compressor blades,turbine and compressor vanes, stator vanes, inlet guide vanes,impellers, propellers, propulsers, aircraft skin materials, medicalimplant devices, and other various and sundry applications too numerousto mention herein.

Other embodiments and advantages of the invention will be apparent fromthe following description, the accompanying drawings and the appendedclaims. While the methods and apparatus described constitutes preferredembodiments of the invention, it is to be understood that the inventionis not limited to the precise method and apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

To provide a more complete understanding of the present invention andfurther features and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic illustration of a preferred embodiment of thedistortion sensing work holder that is the subject of the currentinvention;

FIG. 2 is a schematic illustration of another preferred embodiment ofthe distortion sensing work holder that is the subject of the currentinvention.;

FIG. 3 is a flow diagram of a quality control procedure utilizing adistortion signature.;

FIG. 4 is a flow diagram of a quality control procedure based on theamount of distortion obtained by an operation;

FIG. 5 is a flow diagram for evaluating the proper operation ofmachinery in a treatment operation; and

FIG. 6 is a flow diagram for the real-time evaluation and adjustment ofa treatment process.

DETAILED DESCRIPTION OF THE INVENTION

Although the foregoing invention has been described in some detail forpurposes of clarity of understandings, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, it should be understood that the presentdisclosure is to be considered as exemplary of the principals of theinvention and is not intended to limit the invention to the embodimentsand the specific examples illustrated and the invention is not to belimited to the details given herein, but may be modified within thescope and equivalents of the descriptions and examples contained herein.

Referring to FIG. 1, the work holder 100 for measuring the distortion ofa part 101 is shown. For illustration purposes the part 101 shown is ablading member of a gas turbine engine. Preferably, work holder 100comprises a base 102 integrally formed with an extension 110 and asupport member 104. The support member 104 is standardized such that thework holder 100 may be secured in the tool holder of a conventionalmachine tool (not shown). A securing means 106 is rotatably attached tothe base 102 via the spindle 108. The securing means 106 isinterchangeable and may include vices, clamps, collets, slottedconnectors, and other such means, and is selected based on the geometryof the part being operated on. In the embodiment described herein, thesecuring means 106 is a dovetail slot. The spindle 108 permits thesecuring means 106 to be rotated relative to the base thereby allowingflexibility in the orientation of the part being operated upon.

An elastically deformable and recoverable material member 112, which maybe formed from metal, polymers, composites, or a combination of suchmaterials, and in the form of a beam, rod, plate, and the like, ispreferably flanged at one end, is attached to the extension 110 byfasteners 116 and 120, preferably instrumented bolts. Interposed betweenthe extension 110 and the elastically deformable and recoverable member112 is a spherical pivot point 114 that permits the deflection of theelastically deformable and recoverable member 112 in any directionrelative to the base 102. Strain or deflection sensing means 118,preferably inductive sensors, capacitive sensors, lasers, ultrasonicsensors, mechanical sensors, transducers including LVDT displacementtransducers and non-contact displacement transducers, strain gauges, airaes, instrumented bolts, instrumented washers, electro-optical devices,load cells, and other like means, are attached in various orientationsalong the length of the elastically deformable and recoverable member112. A linkage 122 is fixedly attached to the distal end of theelastically deformable and recoverable material member 112. The linkageextends upward and terminates in a gripping element 124 having opposedgripping faces capable of firmly adhering to the unique contours of thepart being operated upon. Preferably, the gripping faces are adjustableand correspond to the specific geometry of the portion of the part withwhich they contact. Finally, the output of the instrumented bolts 116,120 and the strain or deflection sensing means 118 is fed to means forcapturing, and recording the output of the strain or deflectionmeasuring means. Such means 126 include computers, strip chartrecorders, multi-meters, and other similar means. Preferably, means 126comprises a computer controlled monitoring system capable of collecting,recording, and analyzing the respective output signals.

By way of setup, the part 101, in this example a turbine blade with adovetail connector, is placed in the work holder 100 by first insertingthe dovetail connector into the dovetail slot of the securing means 106.The securing means 106 is then rotated to present the proper profile ofthe part 101 to the operating tool (not shown). With the part 101oriented and clamped in place, the linkage 122 is positioned such thatthe gripping element 124 contacts the surface of the part 101 on the endopposite the securing means 106. The gripping element 124 is thenadjusted so that it firmly grips the edge of the part 101. As atreatment operation is performed on the part 101, any distortionsdeveloped in the part 101 are communicated to the elastically deformableand recoverable beam 112 via the linkage 122. Any distortion of themember 112 is registered by the strain or deflection sensing means 118and/or the instrumented fasteners 116 and 120 and recorded by themonitoring system 126.

Referring to FIG. 2, another embodiment of the distortion sensing workholder is shown. In this embodiment, the work holder has a plurality ofproximity sensors 202 attached to positioning bracket 204 which, inturn, is attached to the distal end of the member 212 and adjustablyarrayed around the part to be treated 201. The proximity sensors 202 areselected from the list including, but not limited to, capacitivesensors, inductive sensors, lasers, ultrasonic sensors, mechanicalsensors, LVDT displacement transducers, non-contact displacementtransducers, strain gauges, or air gauge sensors. The positioningbracket 204 employs means 206 for precisely positioning the sensors 202relative to the part to be treated 201. The positioning means 206 may beselected from the various know means, such as digital micrometers,barrel micrometers, and the like. The proximity sensors 202 arepositioned around the part to be treated so as to be able to measure theparts position in the x, y, and z directions. A sleeve 208 is placedaround the distal end of the part 201 to provide a uniform registrationsurface for interfacing the proximity sensors 202 with the part 201. Theoutput of the proximity sensors 202 is fed to a computer controlledmonitoring system 226 capable of collecting and analyzing the respectiveoutput signals.

By way of operation, any distortions in the part being treated 201 willcause the part to deflect relative to the proximity sensors 202 which,in turn, output a signal corresponding to the magnitude and direction ofthe deflection in each of the x, y, and z directions. These signals arepassed to the monitoring system 226 that records the output of theproximity sensors before, after, and throughout the treatment operation.

Referring now to the flow diagrams of FIGS. 3-6, several differentmethods of using the apparatus of the current invention for qualitycontrol and process monitoring are disclosed. Referring to FIG. 3, inone preferred embodiment, the work holder of the present invention isused for quality control purposes. In step 300, a part is clamped in thework holder and operated on while a real time distortion signature iscollected as indicated in step 302. Thereafter, in step 304, thecollected signature is compared to a known signature for a successfultreatment operation. Step 306 consists of a decision point in which thesuccess of the operation is determined. If there is agreement betweenthe collected and known signatures, the treatment operation has beensuccessful. If there is no agreement between the known and collectedsignatures, then the process proceeds to step 310 where the collectedsignature is analyzed to determine if an appropriate level of agreementcould be reached by further processing. The process is either repeatedto obtain the correct distortion or the part is otherwise discarded asindicated in step 312.

Referring now to FIG. 4, in another preferred embodiment, the part to betreated is clamped in the work holder in step 400 while in step 402 abaseline distortion measurement is obtained for the untreated part toassess the viability of the treatment operation on that particular part.This measurement is compared to similar measurements for known parts. Ifthe part, prior to treatment, contains unacceptable levels of distortionthe process is stopped and the untreated part is discarded. Otherwisethe process proceeds to step 404. In step 404, a treatment operation isperformed. Following the treatment operation, in step 406, a distortionsignature for the treated part is obtained and the total amount ofdistortion due to the operation is subsequently calculated in step 408.The propriety of this amount of distortion is then evaluated in step410. In the next step, step 412, a decision point is reached based onthe amount of distortion that has developed in the part. If the amountof distortion is in an acceptable range, the treatment is acceptable. Ifthe distortion falls outside of the acceptable range, the processproceeds to step 414 where the signature is evaluated to determine iffurther treatment will yield an acceptable distortion.

Referring now to FIG. 5, in another preferred embodiment, the workholder is used to monitor the treatment equipment for faulty operation.In step 502, a distortion signature is collected for a particularoperation while in step 504 this signature is compared to a knownsignature for a verified operation. The outcome of this comparison ispassed to the decision point of step 510. Agreement between the twosignatures causes the process to proceed to step 508, and a properlytreated component is produced. Disagreement between the two signaturescauses the process to proceed to step 510 at which point the process isterminated and the operating parameters of the machine are reassessed instep 514.

Referring now to FIG. 6, in another preferred embodiment of the currentinvention, the work holder is used to monitor individual steps of atreatment operation and thereby adjust treatment parameters to obtainthe desired outcome of the operation. A part is secured in the workholder in step 600. In the next step, step 602, a distortion signatureis collected for a given operation in the treatment process andsubsequently compared to a known distortion signature for the sameoperation in step 604. If the signatures match, the process continues tostep 612 and the next treatment operation is performed as the process isiterated. If the desired degree of agreement between the signatures isnot found, the procedure continues to step 610 where the treatmentparameters are adjusted. The same operation is then conducted on thepart in step 614 and the evaluation of this treatment is again evaluatedbeginning with step 602.

Accordingly, the apparatus and method of the current invention provide arelatively inexpensive and effective system for monitoring thedistortion in a part as a result of residual stresses induced in thepart during treatment operations. By comparing the collected distortionsignatures to known distortion signatures, it is not only possible tomonitor the process for quality control purposes, but to alsoextrapolate the amount and location of residual stresses induced in thepart based on the distortion that occurs.

While the method and apparatus described constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to the precise method and apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

1. A work holder for securing a part during a machining, treatment orother processing operation comprising: securing means for rigidlysecuring a part during processing; a rigid, elastically deformable andrecoverable material member fixedly attached to said securing means; alinkage for mechanically coupling the part being processed and therigid, elastically deformable and recoverable material member; means formeasuring strain developed in or the deflection of said rigid,elastically deformable and recoverable material member; and means forcapturing and recording the output of said strain or deflection sensingmeans.
 2. The work holder of claim 1 wherein said securing means isselected from the group comprising vices, clamps, collets and slottedconnectors.
 3. The work holder of claim 1 wherein said rigid,elastically deformable and recoverable material member is selected fromthe group comprising metals, polymers, and composites.
 4. The workholder of claim 1 wherein said means for mechanically linking the partto said rigid, elastically deformable and recoverable material memberhas a top and bottom end, said bottom end being fixedly attached to therigid, elastically deformable and recoverable material member, and saidtop end comprises means for gripping and releasing the part.
 5. The workholder of claim 4 wherein said means for gripping and releasing the partcomprises gripping surfaces in adjustable opposition to one another,said gripping surfaces having contoured contact faces that correspond tothe specific geometry of the portion of the part with which theycontact.
 6. The work holder of claim 1 wherein said strain or deflectionmeasuring means are selected from the group comprising strain gauges,instrumented bolts, instrumented washers, electro-optical devices, loadcells and transducers.
 7. The work holder of claim 1 wherein said meansfor capturing and recording the output of said strain or deflectionmeasuring means is selected from the group comprising computers, stripchart recorders, or multi-meters.
 8. A work holder for securing a partduring a machining, treatment, or other processing operation comprising:at least one securing means for rigidly securing a part duringprocessing, said securing means being connected to a base, said basebeing integrally formed with a support member for supporting said baseon a conventional tool holder; a rigid, elastically deformable andrecoverable material member having first and second ends, said secondend being proximally fixed to said base of said securing means, and saidfirst end distally suspended away from said base of said securing means;a linkage for mechanically attaching said rigid, elastically deformableand recoverable material member to the part being processed whereby saidlinkage facilitates the communication of distortions from the part tosaid material member and promotes the distortion of said material memberin concert with the part; at least one means for measuring the straindeveloped in or deflection of said material member; and means forcapturing and recording the output of said strain or deflectionmeasuring means.
 9. The work holder of claim 8 wherein said securingmeans comprises clamping surfaces in adjustable opposition to oneanother, said clamping surfaces having contoured contact faces thatfollow the specific geometry of the portion of the part with which theycontact.
 10. The work holder of claim 8 wherein said rigid, elasticallydeformable and recoverable material member is selected from the groupcomprising metals, polymers, and composites.
 11. The work holder ofclaim 8 wherein said rigid, elastically deformable and recoverablematerial member is a beam with a high length to cross-sectional arearatio.
 12. The work holder of claim 8 wherein said means formechanically linking the part to said rigid, elastically deformable andrecoverable material member has a top and bottom end, said bottom endbeing fixedly attached to the rigid, elastically deformable andrecoverable material member, and said top end comprises means forgripping and releasing the part.
 13. The work holder of claim 12 whereinsaid means for gripping and releasing the part comprises grippingsurfaces in adjustable opposition to one another, said gripping surfaceshaving contoured contact faces that correspond to the specific geometryof the portion of the part with which they contact.
 14. The work holderof claim 8 wherein said strain or deflection measuring means areselected from the group comprising strain gauges, instrumented bolts,instrumented washers, electro-optical devices, load cells andtransducers.
 15. The work holder of claim 8 wherein said strain ordeflection measuring means are strain gauges.
 16. The work holder ofclaim 8 wherein said strain or deflection measuring means areinstrumented bolts.
 17. The work holder of claim 8 wherein said meansfor capturing and recording the output of said strain or deflectionmeasuring means is selected from the group comprising computers, stripchart recorders, or multi-meters.
 18. A work holder for securing a partduring a machining, treatment or other processing operation comprising:securing means for rigidly securing a part during processing; means formeasuring the deflection of the part being treated; means forpositioning said deflection measuring means relative to the part beingtreated; and means for capturing and recording the output of saiddeflection measuring means.
 19. The work holder of claim 18 wherein saidsecuring means is selected from the group comprising vices, clamps,collets and slotted connectors.
 20. The work holder of claim 18 whereinsaid deflection measuring means is selected from the group comprisinginductive sensors, capacitive sensors, lasers, ultrasonic sensors,mechanical sensors, LVDT displacement transducers, non-contactdisplacement transducers, strain gauges, and air gauges.
 21. The workholder of claim 18 wherein said means for capturing and recording theoutput of said strain or deflection measuring means is selected from thegroup comprising computers, strip chart recorders, or multi-meters. 22.A work holder for securing a part during a machining, treatment, orother processing operation comprising: at least one securing means forrigidly securing a part during processing, said securing means beingconnected to a base for supporting said base on a conventional toolholder; at least one means for measuring the deflection of said part; atleast one means for positioning the deflection measuring means relativeto the part being treated; and means for capturing and recording theoutput of said deflection measuring means.
 23. The work holder of claim22 wherein said securing means comprises clamping surfaces in adjustableopposition to one another, said clamping surfaces having contouredcontact faces that follow the specific geometry of the portion of thepart with which they contact.
 24. The work holder of claim 22 whereinsaid deflection measuring means is selected from the group comprisinginductive sensors, capacitive sensors, lasers, ultrasonic sensors,mechanical sensors, LVDT displacement transducers, non-contactdisplacement transducers, strain gauges, and air gauges.
 25. The workholder of claim 22 wherein said deflection measuring means are lasers.26. The work holder of claim 22 wherein said strain or deflectionmeasuring means are strain gauges.
 27. The work holder of claim 22wherein said means for capturing and recording the output of saiddeflection measuring means is a computer.
 28. A method for evaluatingthe efficacy of a treatment operation based upon the distortiondeveloped in a part comprising the steps of: clamping the part to betreated in the work holder; performing a treatment operation whilecollecting a distortion signature for the part being operated on;comparing the collected signature to a known, verified signature of thesame operation on the same type of part and assessing if any deviationsbetween the two signatures exist and, if so, whether those deviationsare substantial enough to warrant discarding the treated part.
 29. Themethod of claim 28 further comprising the steps of: acquiring a baselinesignature for the part before the treatment operation is conducted;determining the total amount of distortion occurring in the part as aresult of the operation; and assessing whether the total amount ofdistortion that has developed in the part is reasonable based onestablished tolerances for the part.
 30. A method for assessing theoperation of equipment during a treatment process based on thedistortion developed in a part comprising the steps of: clamping thepart to be treated in the work holder; monitoring the distortion of thepart while the part is treated; comparing, in real time, the distortionsignature of the part to a known and verified signature for a similaroperation; stopping the treatment should substantial deviations existbetween the collected and known distortion signatures; evaluating andadjusting the operational parameters based on the deviation from theknown distortion signature; and reprocessing the part.
 31. A method ofmonitoring and controlling a treatment operation based on the distortiondeveloped in a part comprising the steps of: clamping the part to betreated in the work holder; performing a treatment step while monitoringthe distortion of the part in real time; comparing the distortionsignature for the completed step with known, verified signatures for thesame step; adjusting control parameters based on deviations from theknown, verified signatures; repeating the step to obtain an acceptabledistortion signature; repeating the process for the next step in thesequence.